Method for mapping physical hybrid automatic repeat request indicator channel

ABSTRACT

A method for mapping a physical hybrid automatic repeat request indicator channel (PHICH) is described. The method for mapping a PHICH includes determining an index of a resource element group transmitting a repetitive pattern of the PHICH, according to a ratio of the number of available resource element groups in a symbol in which the PHICH is transmitted and the number of available resource element groups in a first or second OFDM symbol, and mapping the PHICH to the symbol according to the determined index. In transmitting the PHICH, since efficient mapping is performed considering available resource elements varying with OFDM symbols, repetition of the PHICH does not generate interference between neighbor cell IDs and performance is improved.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/406,403, filed on Jan. 13, 2017, which is co-pending U.S. applicationSer. No. 14/726,014, filed on May 29, 2015, which is a continuation ofU.S. application Ser. No. 14/184,345, filed on Feb. 19, 2014 and issuedas U.S. Pat. No. 9,048,991, which is a continuation of U.S. patentapplication Ser. No. 13/012,702, filed on Jan. 24, 2011 and issued asU.S. Pat. No. 8,681,599, which is a continuation of U.S. patentapplication Ser. No. 12/388,243, filed on Feb. 18, 2009 and issued asU.S. Pat. No. 7,894,330, all of which claim priority from and thebenefit of U.S. Provisional Application Ser. No. 61/029,895, filed onFeb. 19, 2008, and Korean Patent Application No. 10-2008-0124084, filedon Dec. 08, 2008, which are all hereby incorporated by reference for allpurposes as it fully set forth herein.

BACKGROUND Field

The present invention relates to a mapping method for frequency andorthogonal frequency division multiplexing (OFDM) symbol regions of asignal transmitted on downlink in a cellular OFDM wireless packetcommunication system.

Discussion of the Background

When transmitting/receiving a packet in a mobile communication system, areceiver should inform a transmitter as to whether or not the packet hasbeen successfully received. If the reception of the packet issuccessful, the receiver transmits an acknowledgement (ACK) signal tocause the transmitter to transmit a new packet. If the reception of thepacket fails, the receiver transmits a negative acknowledgement (NACK)signal to cause the transmitter to re-transmit the packet. Such aprocess is called automatic repeat request (ARQ). Meanwhile, hybrid ARQ(HARQ), which is a combination of the ARQ operation and a channel codingscheme, has been proposed. HARQ lowers an error rate by combining are-transmitted packet with a previously received packet and improvesoverall system efficiency. In order to increase throughput of thesystem, HARQ demands a rapid ACKINACK response from the receivercompared with a conventional ARQ operation. Therefore, the ACKINACKresponse in HARQ is transmitted by a physical channel signaling method.The HARQ scheme may be broadly classified into chase combining (CC) andincremental redundancy (IR). The CC method serves to re-transmit apacket using the same modulation method and the same coding rate asthose used when transmitting a previous packet. The IR method serves tore-transmit a packet using a different modulation method and a differentcoding rate from those used when transmitting a previous packet. In thiscase, the receiver can raise system performance through codingdiversity.

In a multi-carrier cellular mobile communication system, mobile stationsbelonging to one or a plurality of cells transmit an uplink data packetto a base station. That is, since a plurality of mobile stations withinone sub-frame can transmit an uplink data packet, the base station mustbe able to transmit ACKINACK signals to a plurality of mobile stationswithin one sub-frame. If the base station multiplexes a plurality ofACK/NACK signals transmitted to the mobile stations within one sub-frameusing CDMA scheme within a partial time-frequency region of a downlinktransmission band of the multi-carrier system, ACK/NACK signals withrespect to other mobile stations are discriminated by an orthogonal codeor a quasi-orthogonal code multiplied through a time-frequency region.If quadrature phase shift keying (QPSK) transmission is performed, theACK/NACK signals may be discriminated by different orthogonal phasecomponents.

When transmitting the ACK/NACK signals using CDMA multiplexing scheme inorder to transmit a plurality of ACK/NACK signals within one sub-frame,a downlink wireless channel response characteristic should not begreatly varied in a time-frequency region in which the ACK/NACK signalsare transmitted. This is because if orthogonality is maintained betweenthe multiplexed different ACK/NACK signals, a receiver can obtainsatisfactory reception performance without applying a special receivingalgorithm such as channel equalization. Accordingly, the CDMAmultiplexing of the ACK/NACK signals should be performed within thetime-frequency region in which a wireless channel response is notsignificantly varied. However, if the wireless channel quality of aspecific mobile station is poor in the time-frequency region in whichthe ACK/NACK signals are transmitted, the ACK/NACK reception performanceof the mobile station may also be greatly lowered.

Accordingly, the ACK/NACK signals transmitted to any mobile stationwithin one sub-frame may be repeatedly transmitted over separatetime-frequency regions in a plurality of time-frequency axes, and theACK/NACK signals may be multiplexed with ACK/NACK signals transmitted toother mobile stations by CDMA in each time-frequency region. Therefore,the receiver can obtain a time-frequency diversity gain when receivingthe ACK/NACK signals.

However, in a conventional physical hybrid ARQ indicator channel (PHICH)mapping method, there exists a defect that PHICH groups between neighborcells have difficulty avoiding collision as illustrated in FIG. 1.

SUMMARY

An object of the present invention devised to solve the problem lies inproviding a method for mapping a PHICH so that repetition of the PHICHdoes not generate interference between neighbor cell IDs by consideringavailable resource elements varying with OFDM symbols.

The object of the present invention can be achieved by providing amethod for mapping a PHICH, including determining an index of an OFDMsymbol in which a PHICH group is transmitted, determining an index of aresource element group transmitting a repetitive pattern of the PHICHgroup, according to a ratio of the number of available resource elementgroups in the determined OFDM symbol and the number of availableresource element groups in a first or second OFDM symbol, and mappingthe PHICH group according to the determined index.

The PHICH may be transmitted in units of a plurality of PHICH groups,and an index of an OFDM symbol in which an i-th repetitive pattern istransmitted may be defined by the following equation:

$l_{i}^{\prime} = \left\{ \begin{matrix}0 & \begin{matrix}{{{{normal}\mspace{14mu}{PHICH}\mspace{14mu}{duration}},}\mspace{20mu}} \\{{all}\mspace{14mu}{subframes}}\end{matrix} \\i & \begin{matrix}{{{extended}\mspace{14mu}{PHICH}\mspace{14mu}{duration}},} \\{{non}\text{-}{MBSFN}\mspace{14mu}{subframes}}\end{matrix} \\{\left( {\left\lfloor {m^{\prime}/2} \right\rfloor + i + 1} \right){mod}\; 2} & \begin{matrix}{{{extended}\mspace{14mu}{PHICH}\mspace{14mu}{duration}},} \\{{MBSFN}\mspace{14mu}{subframes}}\end{matrix}\end{matrix} \right.$where m′ denotes an index of a PHICH group

The index of the resource element group may be determined according to avalue obtained by multiplying the ratio by a cell ID.

The index of the resource element group may be determined by thefollowing equation:

${\overset{\_}{n}}_{i} = \left\{ \begin{matrix}{\left( {\left\lfloor \left( {N_{ID}^{cell} \cdot {n_{l_{i}^{\prime}}^{\prime}/n_{0}^{\prime}}} \right) \right\rfloor + m^{\prime}} \right){mod}\; n_{l_{i}^{\prime}}^{\prime}} & {i = 0} \\{\left( {\left\lfloor \left( {N_{ID}^{cell} \cdot {n_{l_{i}^{\prime}}^{\prime}/n_{0}^{\prime}}} \right) \right\rfloor + m^{\prime} + \left\lfloor {n_{l_{i}^{\prime}}^{\prime}/3} \right\rfloor} \right){mod}\; n_{l_{i}^{\prime}}^{\prime}} & {i = 1} \\{\left( {\left\lfloor \left( {N_{ID}^{cell} \cdot {n_{l_{i}^{\prime}}^{\prime}/n_{0}^{\prime}}} \right) \right\rfloor + m^{\prime} + \left\lfloor {2{n_{l_{i}^{\prime}}^{\prime}/3}} \right\rfloor} \right){mod}\; n_{l_{i}^{\prime}}^{\prime}} & {i = 2}\end{matrix} \right.$where N_(ID) ^(cell) denotes a cell ID, i denotes an index of arepetitive pattern, n′_(l′) _(i) /n′₀ denotes a ratio between the numberof available resource element groups in an OFDM symbol l′_(i) and thenumber of available resource element groups in a first OFDM symbol, andm′ denotes an index of a PHICH group.

In accordance with another aspect of the present invention, there isprovided a method for mapping a PHICH, including determining an index ofa resource element group transmitting a repetitive pattern of the PHICH,according to a ratio of the number of available resource element groupsin a symbol in which the PHICH is transmitted and the number ofavailable resource element groups in a second OFDM symbol, and mappingthe PHICH to the symbol according to the determined index.

The PHICH may be transmitted in units of a plurality of PHICH groupseach consisting of four resource elements.

The PHICH may be transmitted in units of a plurality of PHICH groupseach consisting of two resource elements.

The index of the resource element group may be determined by thefollowing equation:

${\overset{\_}{n}}_{i} = \left\{ \begin{matrix}{\left( {\left\lfloor \left( {N_{ID}^{cell} \cdot {n_{l_{i}^{\prime}}^{\prime}/n_{1}^{\prime}}} \right) \right\rfloor + m^{\prime}} \right){mod}\; n_{l_{i}^{\prime}}^{\prime}} & {i = 0} \\{\left( {\left\lfloor \left( {N_{ID}^{cell} \cdot {n_{l_{i}^{\prime}}^{\prime}/n_{1}^{\prime}}} \right) \right\rfloor + m^{\prime} + \left\lfloor {n_{l_{i}^{\prime}}^{\prime}/3} \right\rfloor} \right){mod}\; n_{l_{i}^{\prime}}^{\prime}} & {i = 1} \\{\left( {\left\lfloor \left( {N_{ID}^{cell} \cdot {n_{l_{i}^{\prime}}^{\prime}/n_{1}^{\prime}}} \right) \right\rfloor + m^{\prime} + \left\lfloor {2{n_{l_{i}^{\prime}}^{\prime}/3}} \right\rfloor} \right){mod}\; n_{l_{i}^{\prime}}^{\prime}} & {i = 2}\end{matrix} \right.$where N_(ID) ^(cell) denotes a cell ID, i denotes an index of arepetitive pattern, n′_(l′) _(i) /n′₁ denotes a ratio between the numberof available resource element groups in an OFDM symbol l′_(i) and thenumber of available resource element groups in a second OFDM symbol, andm′ denotes an index of a PHICH group.

According to the exemplary embodiment of the present invention,efficiency mapping is performed by considering available resourceelements varying according to OFDM symbols during PHICH transmission, sothat PHICH repetition does not generate interference between neighborcell IDs and performance is improved.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention, illustrate embodiments of the inventionand together with the description serve to explain the principle of theinvention.

In the drawings:

FIG. 1 illustrates an example of a conventional PHICH mapping method;

FIGS. 2 and 3 illustrate resource element groups to which a PHICH ismapped;

FIGS. 4 and 5 illustrate examples of mapping a PHICH when a spreadingfactor is 4;

FIGS. 6 and 7 illustrate examples of mapping a PHICH when a spreadingfactor is 2;

FIGS. 8 to 10 illustrate examples of repetitive mapping of a PHICHapplied to the present invention; and

FIG. 11 illustrates an example of a PHICH mapping method according to anexemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the exemplary embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. The detailed description, which will be given below withreference to the accompanying drawings, is intended to explain exemplaryembodiments of the present invention, rather than to show the onlyembodiments that can be implemented according to the invention.

When transmitting data through downlink of an OFDM wireless packetcommunication system, a channel transmitting ACK/NACK signals may bereferred to as a physical hybrid ARQ indicator channel (PHICH).

In a 3^(rd) generation partnership project (3GPP) long term evolution(LTE) system, the PHICH is repeatedly transmitted three times in orderto obtain diversity gain. Through how many OFDM symbols the PHICH istransmitted is determined depending on information transmitted through aprimary broadcast channel (PBCH) and on whether or not a subframe is formulticast broadcast over single frequency network (MBSFN). If the PHICHis transmitted through one OFDM symbol, the PHICH repeating three timesshould be evenly distributed over a frequency bandwidth of one OFDMsymbol. If the PHICH is transmitted through three OFDM symbols, eachrepetition is mapped to a corresponding OFDM symbol.

FIGS. 2 and 3 illustrate resource element groups (REGs) to which thePHICH is mapped.

Each REG is comprised of four resource elements. Since a first OFDMsymbol includes reference signals RS0 and RS1, locations except for thereference signal locations are available for the resource elements. InFIG. 3, even a second OFDM symbol includes reference signals RS2 andRS3.

FIGS. 4 and 5 illustrate examples of mapping a PHICH when a spreadingfactor (SF) is 4. When an SF is 4, one repetition of one PHICH group ismapped to one REG.

In FIGS. 4 and 5, precoding for transmit diversity is applied. A₁₁, A₂₁,A₃₁, and A₄₁ denote resource elements of an REG constituting a specificPHICH. C₁, C₂, C₃, and C₄ denote resource elements of an REG for PCHICHor a physical downlink control channel (PDCCH). FIGS. 4 and 5 correspondto the cases where the number of antennas is 1 and 2, respectively, whenreference signals are not considered.

FIGS. 6 and 7 illustrate examples of mapping a PHICH when an SF is 2.When an SF is 2, one repetition of two PHICH groups is mapped to oneREG.

Precoding for transmit diversity is applied to FIGS. 6 and 7. FIGS. 6and 7 correspond to the cases where the number of antennas is 1 and 2,respectively, when reference signals are not considered.

In actual implementation as illustrated in FIGS. 2 and 3, it should beconsidered that the number of available REGs in an OFDM symbol includingreference signals is not equal to the number of available REGs in anOFDM symbol which does not include reference signals.

Meanwhile, if a sequence for mapping the PHICH is denoted as y^((p))(0),K,y ^((p))(M_(symb)−1), then y ^((p))(n) satisfies y^((p))(n)=Σy_(i) ^((p))(n), which indicates the sum of all PHICHs in onePHICH group y_(i) ^((p))(n) denotes an i-th PHICH in a specific PHICHgroup. In this case,

z^((p))(i)=

y^((p))(4i),y^((p))(4i+1),y^((p))(4i+2),y^((p))(4i+3)

(where i=0,1,2) denotes a symbol quadruplet for an antenna port p.

An index of a PHICH group has m′=0 as an initial value. A symbolquadruplet z^((p))(i) at m′ is mapped to an REG of (k′,l′)_(i) (wherel′_(i) is an index of an OFDM symbol in which i-th repetition of a PHICHgroup is transmitted, and k′_(i) is an index of a frequency domain).

When a PHICH is transmitted through two OFDM symbols, the PHICH isrepeated twice upon a first OFDM symbol and repeated once upon a secondOFDM symbol according to a transmitted PHICH group. Conversely, thePHICH may be repeated once upon the first OFDM symbol and repeated twiceupon the second OFDM symbol. This may be expressed by the followingEquation 1.

$\begin{matrix}{l_{i}^{\prime} = \left\{ \begin{matrix}0 & \begin{matrix}{{{{normal}\mspace{14mu}{PHICH}\mspace{14mu}{duration}},}\mspace{20mu}} \\{{all}\mspace{14mu}{subframes}}\end{matrix} \\i & \begin{matrix}{{{extended}\mspace{14mu}{PHICH}\mspace{14mu}{duration}},} \\{{non}\text{-}{MBSFN}\mspace{14mu}{subframes}}\end{matrix} \\{\left( {\left\lfloor {m^{\prime}/2} \right\rfloor + i + 1} \right){mod}\; 2} & \begin{matrix}{{{extended}\mspace{14mu}{PHICH}\mspace{14mu}{duration}},} \\{{MBSFN}\mspace{14mu}{subframes}}\end{matrix}\end{matrix} \right.} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$

In Equation 1, l′_(i) denotes an index of an OFDM symbol in which i-threpetition of a PHICH group is transmitted, m′ denotes an index of aPHICH group, and i denotes the number of repetitions of a PHICH. Whenthe PHICH is repeated three times, i has values of 0, 1, and 2.

FIGS. 8 to 10 illustratively show Equation 1.

FIGS. 8 and 9 show the cases where l′_(i)=0 and l′_(i)=(└m′/2┘+i+1)mod2,respectively. FIG. 10 shows the case where l′_(i)=1 and a PHICH group isrepeated at a PHICH duration of 3.

A PHICH, which is an important channel for transmitting ACK/NACK signalsindicating whether or not data has been received, should be transmittedas stably as possible. Further, since ACK/NACK signals should betransmitted to a user even in a cell edge, substantial power is usedcompared with other channels. If locations for transmitting the PHICHsin respective cells are the same, PHICH transmission performance may bedeteriorated due to interference caused by transmission of the PHICHbetween neighbor cells. Accordingly, if transmission locations of thePHICH in respective cells differ, interference caused by transmission ofthe PHICH between neighbor cells is reduced. Consequently, PHICHtransmission performance can be improved. Namely, if mapping locationsof the PHICH are determined according to cell IDs, the above-describedproblem can be solved. The PHICH is repeatedly transmitted three timesto obtain diversity gain. To increase the diversity gain, eachrepetition should be evenly distributed over an entire frequencybandwidth.

To satisfy the above conditions, a PHICH group is transmitted in unitsof an REG consisting of 4 resource elements. The location of atransmission start REG of the PHICH is designated according to a cell IDand each repetition of the PHICH is arranged at an interval of a valueobtained by dividing the number of REGs which can be transmitted by 3based on the transmission start REG. However, when such a repetition ofthe PHICH is distributed over a plurality of OFDM symbols, the number ofREGs which can be used for PHICH transmission in each OFDM symboldiffers. That is because, in the first OFDM symbol, a physical controlformat indicator channel (PCFICH) for transmitting information includingthe number of OFDM symbols used for a control channel is transmitted,and because reference signals transmitted in the first and second OFDMsymbols differ according to the number of transmit antennas. When thePHICH is transmitted through multiple OFDM symbols including differentREGs, since the number of REGs in each OFDM symbol differs, repetitionsof each PHICH are not evenly dispersed over an entire frequencybandwidth. The location of the first REG should be designated accordingto a cell ID and a repetitive pattern should be allocated at regularintervals based on an index of the first REG However, since resolutionof a frequency location depending on the index differs according to thenumber of REGs in each OFDM symbol, is there exists a defect that areference location is changed.

Therefore, when the PHICH is transmitted through multiple OFDM symbols,if the start location according to the cell ID is determined inconsideration of a ratio of REGs of the first start symbol to REGs ofthe other symbols, the above problem can be solved. When the PHICH istransmitted through one or three OFDM symbols, the location of the firststart symbol is always the first OFDM symbol. However, when the PHICH istransmitted through two OFDM symbols, the first PHICH group is startedfrom the second OFDM symbol. Accordingly, if the ratio of REGs isconsidered, a reference symbol should be changed.

The above description may be expressed by the following equation 2.

$\begin{matrix}{{\overset{\_}{n}}_{i} = \left\{ \begin{matrix}{\left( {\left\lfloor \left( {N_{ID}^{cell} \cdot {n_{l_{i}^{\prime}}^{\prime}/n_{0}^{\prime}}} \right) \right\rfloor + m^{\prime}} \right){mod}\; n_{l_{i}^{\prime}}^{\prime}} & {i = 0} \\{\left( {\left\lfloor \left( {N_{ID}^{cell} \cdot {n_{l_{i}^{\prime}}^{\prime}/n_{0}^{\prime}}} \right) \right\rfloor + m^{\prime} + \left\lfloor {n_{l_{i}^{\prime}}^{\prime}/3} \right\rfloor} \right){mod}\; n_{l_{i}^{\prime}}^{\prime}} & {i = 1} \\{\left( {\left\lfloor \left( {N_{ID}^{cell} \cdot {n_{l_{i}^{\prime}}^{\prime}/n_{0}^{\prime}}} \right) \right\rfloor + m^{\prime} + \left\lfloor {2{n_{l_{i}^{\prime}}^{\prime}/3}} \right\rfloor} \right){mod}\; n_{l_{i}^{\prime}}^{\prime}} & {i = 2}\end{matrix} \right.} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack\end{matrix}$

In Equation 2, n _(i) denotes an index of an REG in which a repetitivepattern₀of each PHICH is transmitted, N_(ID) ^(cell) denotes a cell ID,n′_(l′) _(i) denotes the number of REGs which can be used for PHICHtransmission in an OFDM symbol l′_(i), n′_(l′) _(i) /n′₀ denotes a ratiobetween the number of available resource element groups in an OFDMsymbol l′_(i), and the number of available resource element groups in afirst OFDM symbol and is a parameter for solving a problem caused by thedifferent number of REGs between symbols, and m′ denotes an index of aPHICH group as indicated in Equation 1. m′ is desirably increased by 1.

FIG. 11 illustrates an example of a PHICH mapping method according to anexemplary embodiment of the present invention. As illustrated in FIG.11, PHICH resource collision can be avoided based on cell planning.

If the PHICH is mapped from the second OFDM symbol, n′_(l′) _(i) /n′₀ ischanged to n′_(l′) _(i) /n′₁. This may be expressed by the followingEquation 3.

${\overset{\_}{n}}_{i} = \left\{ \begin{matrix}{\left( {\left\lfloor \left( {N_{ID}^{cell} \cdot {n_{l_{i}^{\prime}}^{\prime}/n_{1}^{\prime}}} \right) \right\rfloor + m^{\prime}} \right){mod}\; n_{l_{i}^{\prime}}^{\prime}} & {i = 0} \\{\left( {\left\lfloor \left( {N_{ID}^{cell} \cdot {n_{l_{i}^{\prime}}^{\prime}/n_{1}^{\prime}}} \right) \right\rfloor + m^{\prime} + \left\lfloor {n_{l_{i}^{\prime}}^{\prime}/3} \right\rfloor} \right){mod}\; n_{l_{i}^{\prime}}^{\prime}} & {i = 1} \\{\left( {\left\lfloor \left( {N_{ID}^{cell} \cdot {n_{l_{i}^{\prime}}^{\prime}/n_{1}^{\prime}}} \right) \right\rfloor + m^{\prime} + \left\lfloor {2{n_{l_{i}^{\prime}}^{\prime}/3}} \right\rfloor} \right){mod}\; n_{l_{i}^{\prime}}^{\prime}} & {i = 2}\end{matrix} \right.$

In Equation 3, N_(ID) ^(cell) denotes a cell ID, i denotes an index of arepetitive pattern, n′_(l′) _(i) /n′₁ denotes a ratio between the numberof available resource element groups in an OFDM symbol l′_(i) and thenumber of available resource element groups in a second OFDM symbol, andm′ denotes an index of a PHICH group. As in Equation 2, m′ is desirablyincreased by 1.

Meanwhile, the location of the first PHICH group is allocated and thenthe other PHICH groups may be mapped successively after the first PHICHgroup.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the spirit or scope of the invention. Thus, it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

The present invention provides a mapping method for frequency and OFDMsymbol regions of a signal transmitted on downlink in a cellular OFDMwireless packet communication system and may be applied to a 3GPP LTEsystem, etc.

What is claimed is:
 1. A method for mapping a physical hybrid automaticrepeat request indicator channel (PHICH) to at least one orthogonalfrequency division multiplexing (OFDM) symbol, each OFDM symbolcomprising a plurality of resource element groups, each resource elementgroup being comprised of four resource elements, the method comprising:determining indexes of resource element groups in which the PHICH istransmitted; and mapping the PHICH to at least one OFDM symbolsaccording to the determined indexes, wherein said indexes are determinedaccording to ratio n′_(l′i)/n′₀ or ratio n′_(l′i)/n′₁ in OFDM symbol,having index l′_(i), n′_(l′i) is the number of available resourceelement groups in the OFDM symbol, having index l′_(i),n′₀ is the numberof available resource element groups in OFDM symbol, having index 0, ofa sub-frame, n′₁ the number of available resource element groups in OFDMsymbol, having index 1, of the sub-frame, available resource elementgroups in the OFDM symbol, having index l′_(i), are resource elementgroups which can be used for PHICH transmission in the OFDM symbol,having index l′_(i).